Screw extruder with improved mixing pins

ABSTRACT

An extruder, in particular for elastomer material, is described, said extruder being of the type comprising an extruder body ( 1 ) provided with a cylindrical chamber ( 2 ) inside which an extrusion screw ( 3 ) is rotatably inserted, and a plurality of mixing pin members ( 4 ) inserted in the casing of the cylindrical chamber ( 2 ) and projecting inside the said chamber ( 2 ), wherein each pin member ( 4 ) is arranged tangential to the core of said screw ( 3 ), opposite interruptions in thread of the screw ( 3 ) and passes through the whole of the chamber ( 2 ), engaging in both the opposite walls of said casing.

[0001] The present invention relates to an extruder provided with mixingpins.

[0002] As is known, screw extruders are generally composed of anextruder body with a cylindrical chamber containing, longitudinally, ahelical screw which, when rotated, pushes, mixes and compresses theextrusion material towards an extrusion head on which a die is mounted.

[0003] According to the prior art it is also envisaged, in certaincases, that the cylindrical chamber contains mixing pins or rods. As canbe seen from FIGS. 1A and 1B, which show an extrusion cylinder accordingto the prior art, the pins are normally arranged at regular intervals inthe longitudinal direction and inserted radially in the casing of theextruder body.

[0004] In order to prevent interference during operation, the pins arefit in the casing walls, opposite interruptions in the thread of theextrusion screw, and also project inwardly the cylindrical chamber by adistance which is undoubtedly less than the transverse dimension of thegap between the core of the screw and the internal surface of the saidchamber.

[0005] The main functions of the pins is that of creating an obstacle tothe feeding movement of the extrusion material, which is thus disturbedand mixed more thoroughly, and of exchanging heat with extrusionmaterial.

[0006] However, at least two types of problem are associated with thisconfiguration.

[0007] A first problem is linked with the impossibility of correctlyregulating the temperature of these pins.

[0008] U.S. Pat. No. 5,324,108 discloses a pin cylinder extruder whereinsome pins are provided tangentially in the chamber. Such pins have aninternal dead hole where thermal fluid is made to circulate back andfort. Such a kind of circulation path for the thermal fluid is notefficient and requires that the internal diameter of the pins besufficiently large to accomodate an exit tube.

[0009] Moreover, in the event of one or more of the rods breaking, thefragments which break off end up inside the cylindrical chamber,contaminating the extrusion material and, in the worst instance,damaging the extrusion screw or the actual cylindrical chamber.

[0010] U.S. Pat. No. 5,480,227 discloses a screw extruder provided withsome diagonally inclined pins. The wall of the cylinder receiving thescrew has an anular recess. Pins are located very close to a bead of thescrew and partially embedded in said cylinder wall. In this way, theextruded material may come into contact with a small portion of thelateral surface of the pins, resulting in a poor thermal exchange.Moreover, extruded material is chennelled between the pins but notactually mixed.

[0011] The object of the present invention is to overcome the problemsencountered in the prior art by providing a new and original design ofextruder in which the temperature of the pins can be easily controlledeven with small diameters, the extrusion material may be intimatelymixed and thermally regulated by the pins and it is possible to preventdamage resulting from possible breakage of these pins.

[0012] These objects are achieved by means of a screw extruder asdescribed in claim 1.

[0013] Further characteristic features and advantages of the deviceaccording to the invention will emerge, however, more clearly from thefollowing detailed description provided by way of example andillustrated in the accompanying drawings, in which:

[0014]FIG. 1A is a longitudinally sectioned view through an extruderbody according to the prior art;

[0015]FIG. 1B is a cross-sectional view taken along the line I-I of FIG.1A;

[0016]FIG. 2 is a partial longitudinal section through an extruder bodyaccording to the invention; and

[0017]FIG. 3 is a cross-sectional view along the line III-II in FIG. 2.

[0018] An extruder comprises, in a manner known per se, an extruder body1 containing a cylindrical chamber 2 which longitudinally houses anextrusion screw 3.

[0019] The extrusion screw 3 is composed of a core N and a thread Fwhich is spirally wound around the core N with a constant or variablepitch depending on the circumstances.

[0020] Rotation of the screw, which is obtained by means of knownactuating means (not shown), causes feeding and compression of anextrusion material (in particular rubber, elastomer material or thelike) introduced into the cylindrical chamber via a suitable loadingopening (not shown).

[0021] According to the invention, mixing pin members 4 which passthrough the whole of the thickness of the facing walls of the extruderbody 1 and the cylindrical chamber 2 are provided. The pin members 4 areinserted and fixed inside suitable insertion holes in the extruder bodyso as to be tangential, and very close, to the core N of the extrusionscrew 3.

[0022] Each pin member is composed of a hollow rod 4 a which has, fixedto its ends, locking bushes 4 b (for example engaged with the respectiverod by means of a thread) by means of which the pin members are lockedtightly on the extruder body.

[0023] At the point where the chamber 2 is passed through by one of therods 4, the thread F has an interruption Fi.

[0024] Preferably, the rods are arranged in pairs on either side of thecore N. Moreover, the pairs of rods are distributed longitudinally,alternating at 90° with respect to each other (FIG. 2).

[0025] The rods 4 a are tubular, namely they have at least one internalflow channel through which a heat-regulating liquid (for example oil orhot water or a cooling fluid, depending on the circumstances) can becirculated.

[0026] As shown schematically in FIG. 3, the fluid is circulated byintroducing it at one end of the hollow rod 4 a and allowing it flow outat the opposite end by means of suitable delivery and return pipes (notshown).

[0027] Further, as can be well appreciated from FIG. 3, between the rods4 a and the internal surface of the cylinder wall a substantialclearance 5 is left. In other words, at least a portion of said rods iswholly wetted by the extrusion material. This allows the extrusionmaterial to pass all around the pin members, so as to contact a majorportion thereof and enhance the thermal exchange effect. Moreover, thetwo extrusion material streams, coming from the two sides of the rod 4 aduring extrusion, meet each other downstream of the rod 4 a resulting inan enhanced mixing effect.

[0028] The rods according to the invention allow a continuous flow of aheat-regulating fluid, such that it is possible to control in an optimummanner the temperature in accordance with the specific requirements.Moreover, since they are fixed to the extruder body at both ends, evenif the portion inside the cylindrical chamber should break, theynevertheless would remain attached to the extruder body without theirfragments being able to end up inside the chamber.

[0029] This latter characteristic feature is of considerable importanceand means that major savings may be made with regard to repair andmaintenance costs.

[0030] It is understood, however, that the scope of protection of theinvention described above is not limited to the particular embodimentshown, but that it is extended to any other constructional modificationwhich achieves the same usefulness.

[0031] For example, although mixing pin members with a circularcross-section have been described, it is also possible to use, inparticular circumstances, tubular rods with a differently profiledcross-section.

1. Extruder of the type comprising an extruder body with a cylindricalchamber inside which an extrusion screw is rotatably inserted, aplurality of mixing thermally regulated pin members being insertedthrough a casing of said cylindrical chamber and being fixed with theirrespective ends to the wall of said extruder, characterized in that atleast a portion of said rods projects inside said cylindrical chamber insuch a way that its entire circumferential surface may be wetted by theextrusion material so as to enhance heat exchange and mixing effect. 2.Extruder according to claim 1, in which said pin members comprise anhollow rod and a heat-regulating fluid is circulated inside them. 3.Extruder according to claim 2, in which said rods consist of a tubularrod, the ends of which emerge externally from said extruder body and arefixed thereon by means of fixing bushes.
 4. Extruder according to anyone of the preceding claims, in which said pin members are provided inpairs, one on one side and one on the opposite side of the extrusionscrew.
 5. Extruder according to claim 4, in which said pairs of pinmembers are arranged longitudinally, alternating at 90° with respect toeach other.